The disclosure relates to the field of electronic communications. More particularly, the disclosure relates to encoding of variable sized data frames in an electronic communication system.
Time Division Multiple Access (TDMA) or Multi-Frequency TDMA (MF-TDMA) are commonly used techniques to support multiple access in mesh networks and in the inbound traffic for Hub spoke networks. The transmission of continuously variable size data frames, such as Internet Protocol (IP) datagrams, over a TDMA network requires the packaging of the data into messages, herein described as MAC (Medium Access Control) messages or MAC frames, each suitable for transmission within a single RF burst. The MAC message in this case may comprise a number of IP datagrams, a single IP datagram, a fraction of an IP datagram, and/or control messages. RF bursts in a wireless data transmission each consist of a number of slots, and the slot size may be chosen as small as one data symbol in length. A practical wireless data transmission may thus have a large number of possible burst lengths (measured in slots) depending on the nature of the network traffic.
Historically, the RF burst has been created by encapsulating the data (placing it into a lower level networking layer and adding headers, etc. as needed) into fixed size containers, commonly called ‘cells’. When the size of the datagram is larger than the size of the cell, the datagram must be fragmented into smaller pieces, each of which will fit into one cell. Additional overhead must be added to enable the correct reassembly of the MAC message from the cells. This is how one aspect of Digital Video Broadcasting-Return Channel via Satellite (DVB-RCS, ETSI Standard EN 301 790) works, with a fundamental cell size of 48 bytes (53 Byte ATM cell with 48 bytes of usable payload). An adaptation layer (AAL-5) is used to allow datagram re-assembly. The same concept could be employed with a different cell size, however optimal selection of the cell size is not obvious.
Small cell sizes are efficient for the transport of small messages but produce a lot of overhead for the transport of large messages. A large cell size provides for the efficient transport of large messages, however are very inefficient for the transport of messages which are much smaller than the cell size since the remainder of the cell must be padded with zeros (alternatively referred to as zero filling).
A similar problem is exists with selection of the Forward Error Correction (FEC) encoding used on the physical layer. In particular, the selection of the FEC block size is a critical parameter. Information theory predicts that the reliable transport of long block size codes can be performed at a lower Eb/No than short block size codes. For example, the Random Coding Bound (RCB) establishes that an IP datagram of length 1500 bytes can be transmitted at a rate of 1 bit per symbol, with a block error rate of 1E-4, using an Eb/No of 0.25 dB. But a minimum sized IPV4 frame of 40 bytes (assuming TCP/IP) needs an Eb/No of 1.35 dB to transmit at the same rate with the same reliability (BLER=1E-4). The difference is 1.1 dB. Practical FEC codes generally deviate from the RCB much more so at short block sizes than large block sizes, so this difference becomes even larger with practical coding systems. Furthermore, many systems frequently need to transmit very small sized MAC management messages, for things such as bandwidth request, ranging, and other management information. It is common for such messages to be smaller than the minimum sized IP frame. The minimization of transmit power requirements argues for the selection of a large FEC block size. However, small IP datagrams and Media Access Control (MAC) messages will be transmitted very inefficiently due to the required zero padding to fill the code block. Smaller FEC code block selection reduces the inefficiency problem but now requires more transmit Effective Isotropic Radiated Power (EIRP).
A brute force approach to provide the most efficient use of the communication channel capacity is to simply encapsulate each MAC message into a burst that contains exactly one encoded block of data that perfectly matches its input code size to the MAC message and its output code size to the minimum slot length burst. This technique has the severe disadvantage of requiring a different encoder/decoder pair for each required length of RF burst, as measured in slots. As well, some method must be employed by both transmitter and receiver in order to coordinate the appropriate decoder to match the selected encoder.